This invention relates to abrasive products and specifically to coated abrasives and a process for making such products.
In the production of coated abrasives the conventional technique employed is to coat a substrate with a curable maker coat and then to apply abrasive grits to the maker coat before it has become cured such that the grits are retained by the maker coat and are thereby anchored to the substrate. A size coat is conventionally applied over the grits to provide secure anchorage while the coated abrasive is actually in use. To enhance the performance of the abrasive grits, especially in the grinding of metals such as steels, it is often conventional to apply over the size coat a supersize coat comprising a binder and a grinding adjuvant. This adjuvant can be a lubricant or an antistatic additive to reduce loading of the coated abrasive during use. More commonly however the grinding adjuvant is a "grinding aid" which decomposes during use and the decomposition products of which facilitate removal of metal from the workpiece. The grinding adjuvant, to be most effective, should generally be located at the point of grinding, as close as possible to the point at which the abrasive grit contacts the metal workpiece.
The abrasive grits are conventionally applied to the maker coat using an electrostatic technique in which the grits are projected towards the maker coat. This application technique tends to align the grits such the longest dimension is perpendicular to the plane of the backing when the grit is anchored in place. This arrangement is very advantageous to the finished coated abrasive since it presents the smallest surface area of grit to the workpiece and maximizes the applied force per grit and therefore the effectiveness of the abrading process at a given power output.
In some respects however this can be a disadvantage since, if the grits have a weak shape, (defined as having a ratio of the longest dimension to the largest dimension perpendicular to the longest dimension, or "aspect ratio",) greater than about 1.5 the supersize layer tends to collect in the spaces between the grits and thus be removed from the grit tips, unless unusually large amounts of size coat and/or supersize coat are used.
In a conventional process for the manufacture of coated abrasives, a substrate or backing layer is prepared and then treated with a coat of a maker resin and a layer of abrasive grits is deposited thereon. The maker coat is then at least partially cured and a further binder coat, referred to as a size coat, is applied over the abrasive grits. With radiation cured binders, the cure of the maker coat is typically completed before application of the size coat.
The abrasive grits are applied either by gravity coating or by an electrostatic process in which the grits are impelled towards the surface to be coated by electrostatic forces. This electrostatic coating technique is referred to as the UP coating technique.
It has been discovered that, with premium aggressively cutting grits particularly, a closed coat, (that is a coat with the maximum amount of grit that can be deposited on a surface in a single layer), can lead to burning of the surface of the workpiece. Maximum efficiency is obtained when the load per active abrading grit is maximized during grinding and the cutting grits are spaced to give the workpiece an opportunity to cool between abrading events. One solution to this problem is proposed by U.S. Pat. No. 5,011,512 which teaches the incorporation of non-abrasive particles with a Knoop hardness less than about 200 along with the abrasive grits. The abrasive grits and non-abrasive particles were of the same size and the non-abrasive particles appear to space the abrasive grits allowing them to cut more efficiently. The "spacing" concept is often described in terms of "percent closed coat". This is calculated by measuring the amount of abrasive grits required to provide a monolayer coverage of a unit amount of the substrate and expressing the actual amount of abrasive particles applied per unit area as a percentage of the amount required to deposit a monolayer. Very similar teaching regarding spacing of abrasive grits using friable fillers is found in U.S. Pat. No. 1,830,757; U.S. Pat. No. 3,476,537; and EP 0 494,435-A1.
Efficiency of cutting is conventionally enhanced by the use of a supersize additive in the last applied layer of a coated abrasive. However a problem is encountered with abrasive grits of a weak shape. All grits with an aspect ratio greater than about 1.5 are described generically as having a "weak" shape. If these stand perpendicular to the surface to which they are bonded, (as is generally preferred), the cutting surface is far removed from the bulk of the supersize-containing layer. This problem can be solved by addition of very large amounts of size and supersize such that the spaces between the grits is filled up by the supersize formulation. However as the shapes get "weaker", this approach becomes much more expensive.
Weak shaped abrasive grits are obtainable by crushing larger particles using a rolls crusher. These however, while predominantly "weaker" in shape than impact crushed abrasive grain, do not in general have more than about 20% of the particles with an aspect ratio of more than 2:1.
In recent years a new form of grit has been developed that has a filamentary particle form with a substantially uniform cross-sectional shape and a length dimension perpendicular to that cross-section that is at least equal to, and more usually much larger than, the greatest dimension of the cross-section. Such grits will have the appearance of rods or cones or square-based pyramids for example.
One form of such grits is made from a sol-gel alumina that has been shaped into a filamentary particle shape before it is dried and fired to produce a remarkably effective abrasive grit. Such grits are described in U.S. Pat. No. 5,009,676 and coated abrasives made using them are described in U.S. Pat. No. 5,103,598. Another form of grits that is particularly well suited for use in the present invention are grits with a very weak shape but not necessarily having a uniform cross-sectional shape. "Weak" but non-uniform shapes are conventionally produced using a roll-crushing comminution technique. These have an aspect ratio somewhat greater than 1 but have very few grits with aspect ratios greater than 2:1, (usually less than 20%). However it has been found that explosive comminution of materials containing volatilizable material that form ceramics when fired yields much weaker shapes than are achievable using the conventional roll-grinding techniques. The production of such grits is described in U.S. Pat. No. 5,725,162. To the extent that they share the problems described above, these weak-shaped abrasive grits can also be used in the present invention.
With very weak-shaped grits, a very significant moment is developed, (which increases with the "weakness"), upon contact with a workpiece under abrading conditions. This can lead to premature fracture of the grit or even displacement from the backing of the whole grit. This could be cured by addition of a thicker size coat which would also solve the issue of the location of the supersize additive in the coating. However, as indicated above, this becomes very expensive and can also result in delays in curing and perhaps differences in extent of cure throughout the thickness of the size layer.
The present invention provides a novel way of overcoming the problem of grinding aid efficiency by permitting the placing of the grinding aid formulations at the point of maximum utility without the use of excessive amounts of the size or supersize formulations.
The problem also provides a way of ensuring that very weak shaped grits wear at a more uniform rate by ensuring that they are more securely anchored without the use of greater volumes of size coat than would be economic.